(n+2)!/n!=56; n ∈ N
(n+2)(n+1)•n!/n!=56
(n+2)(n+1)=8•7 =>n=6
sau n²+3n+2-56=0
n²+3n-54=0
Δ=9+216=225
n1=(-3+15)/2=6
n2=(-3-15)/2=-9, nu convine
=> n=6
(n+2)!/n!=56
(n+2)×(n+1)×n!/n!=56 - simplificam cu n!
(n+2)×(n+1)=56
(n+2)×(n+1)=8×7
n+2=8 ⇒ n=8-2 ⇒ n=6
n+1=7 ⇒ n=7-1 ⇒ n=6
